Automated Mineralogy (AM) systems, such as the QEMSCAN and MLA from FEI Company, have been used for many years to determine minerals present in mines in order to determine the presence and distribution of valuable minerals. Such systems direct an electron beam toward the sample and measure the energy of x-rays coming from the material in response to the electron beam. One such process is called “energy dispersive x-ray analysis” or “EDS,” which can be used for elemental analysis or chemical characterization of a sample.
In EDS analysis, a high-energy beam of charged particles such as electrons or protons, or a beam of x-rays, is focused into the sample being studied to stimulate the emission of x-rays from the sample. The energy of the x-rays emitted from a specimen is characteristic of the atomic structure of the elements making up the specimen. By measuring the number and energy of the x-rays emitted from a specimen using an energy-dispersive spectrometer, and comparing the measured spectra to a library of reference spectra of known compositions, the unknown elemental composition of the specimen can be determined. EDS analysis, especially when coupled with back-scattered electron (BSE) analysis, can also be used to quantify a wide range of mineral characteristics, such as mineral abundance, grain size, and liberation. Mineral texture and degree of liberation are fundamental properties of ore and drive its economic treatment, making this type of data invaluable to geologists, mineralogists and metallurgists who engage in process optimization, mine feasibility studies, and ore characterization analyses.
Mineral analysis systems of this type are also used in the oil and gas industry, as well as in mining. Drill cuttings (drill bit-induced rock chips) and diamond drill cores can be analyzed to allow geologists to determine the exact nature of the material encountered during drilling, which in turn allows more accurate predictions as to the material still ahead of the drill, thus reducing risk in exploration and production. During drilling, a liquid referred to as “mud” is injected into the well to lubricate the drill and return the cuttings out of the well. A sample can be taken from the mud that includes cuttings from the drill. Great importance is often placed on documenting cuttings and cores as accurately as possible, both at the time of drilling and post-drilling. Characterizing down-hole lithological variation in a reservoir sequence is a critical requirement in exploration and production wells, and mineralogical and petrographic studies underpin the fundamental understanding of reservoir and seal characteristics. Traditional optical microscopes, scanning electron microscopes (SEM), electron probe microanalyzer (EPMA) and x-ray diffraction (XRD) analysis methods are well established and widely used within the industry.
A problem inherent in Automated Mineralogy is how to obtain representative, useful, accurate, and precise three-dimensional microscopic quantitative knowledge about a huge three dimensional macroscopic population by means of tiny two dimensional microscopic samples. One of the most important considerations for this type of analysis is whether the prepared sample being analyzed is truly a representative sample. For this reason, sample preparation techniques are particularly important for meaningful analysis. Samples suitable for use in analytical instruments such as QEMSCAN and MLA systems should be prepared so that the material to be analyzed can be presented to the instrument as a flat, carbon coated surface. Typically, material to be analyzed, such as material retrieved from a mine, is carefully sampled from the mine, crushed, and mixed with epoxy in a mold. The sample mold is cured and then the sample is removed. The sample is ground to expose the interior of some of the particles, and then polished to produce a smooth surface. The surface is coated with a carbon film to form a conductive coating to prevent electrical charging by the electron beam, and the sample is typically observed using a camera to ensure that it was properly prepared before it is inserted into the vacuum chamber of the electron beam system.
To ensure that the results of the analysis are representative, the particles have to be uniformly distributed in the epoxy, so that when the sample is ground, the probability of exposing all particles is uniform. Using prior art methods of sample preparation, the process of preparing a suitable representative sample typically takes about 8 hours. This has long been considered acceptable in the mining industry. In the drilling industry, it would be desirable to obtain much faster feedback regarding the composition of the drill cutting in order to adjust the drilling process sooner.
Accordingly, what is needed is a method and apparatus for preparing suitable representative samples for EDS analysis, or other similar types of analysis, that allows the samples to be produced much more rapidly, preferably in less than one hour as opposed to the eight hour time frame of the prior art.